Inkjet printing systems are generally of two types: DOD printing systems and CIJ printing systems. Inkjet printing is a standard method for printing a colored image onto a substrate wherein a stream of ink droplets is directed from a printing device to a surface of a suitable receiver element or substrate. The direction of the stream of droplets is controlled electronically in causing the droplets to print the desired image or information on the substrate surface without requiring contract between the printing device and the surface to which the ink is applied. Objects comprising substrates to which inkjet printing is well suited include but are not limited to, containers for consumer products, currency, draft checks, envelopes, letterhead, documents of various types, identification cards, lottery tickets, bank cards, identification strips, labels, brochures, signage, and other well-known materials.
Drop-on-demand printing systems are widely used in home or consumer inkjet printers and slower consumer printers, both of which have been available for several decades. As the name implies, this type of inkjet printing uses a printhead that ejects drops of ink only when signaled to do so by a digital controller.
CIJ printing systems generally comprise two main components, a fluid system (including an ink reservoir) and one or more printheads. Ink can be pumped through a supply line from the ink reservoir to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s), under sufficient pressure to cause ink streams to issue from the orifices of the printhead(s). Stimulations can be applied to the printhead(s) to cause those ink streams to form streams of uniformly sized and spaced drop, which are deflected in a suitable manner, into printing or non-printing paths. Non-printing drops can be returned to the ink reservoir using a drop catcher and a return line. Thus, in contrast to DOD printing systems, CIJ printing systems involves use of a continuous stream of ink drops that are separated to discriminate between spaced printing drops and non-printing drops. This discrimination can be accomplished by electrostatically charging the drops and passing the charged drops through an electric field. Charged drops are deflected by a charge field and can be caught and returned to the reservoir of ink. Uncharged drops are printed onto a substrate or receiver material. This discrimination can also be accomplished when the printhead digitally creates large and small drops. Small drops are deflected by an air current and returned to the reservoir of ink. Large drops, being less affected by the air current, are printed onto a substrate or receiver material. Some useful CIJ printing apparatus and printhead fabrication are described for example in U.S. Pat. No. 6,588,888 (Jeanmaire et al.) and U.S. Pat. No. 6,943,037 (Anagnostopoulos et al.).
In general, such pigment-based colored inks can comprise a wide variety of colored organic pigments that can be chosen depending upon the specific application and performance requirements for the printing system and desired printing results (for example, desired hue). For example, such organic or inorganic pigments can include but are not limited to, carbon black or other black pigments, red pigments, green pigments, blue pigments, orange pigments, violet pigments, magenta pigments, yellow pigments, and cyan pigments. The printed images using such pigment-based inks are generally desired to have a visual density of at least 0.5.
There are some commercially available green organic pigments that may be useful in inkjet ink formulations. Such green pigments can include copper phthalocyanine pigments having a copper phthalocyanine skeleton that are generally blue-green in hue. While it has been desired to formulate such green pigments into inkjet ink compositions for use in CU, it is highly important to keep the green pigment particle size very small, for example, where the median particle size (50% intensity mode) is less than 100 nm or even less than 85 nm, and at least 95% (intensity mode) of the particles have a particle size of less than 150 nm.
Yet, it has been difficult to mill the noted green organic pigments to such small particle sizes for incorporation into the inkjet ink compositions. When attempts were made to mill common green pigments to the desired particle size, the particle size is initially reduced but as milling continues, the particle size unexpectedly increases from agglomeration of the smaller particles. It became apparent that there is a need to effectively stabilize the green organic pigment particle size and to prevent particle agglomeration especially during the milling operation.
Moreover, there is a need to expand color gamut of green organic pigments beyond that possible with the pigments used alone while keeping the green organic pigments from premature agglomeration.